Patent document 1 discloses a method of optically writing, reading, and/or deleting information on a conventional multilayer optical information recording medium having at least two recording layers and two guide layers; and an apparatus for writing, reading, and/or deleting information on the conventional multilayer optical information recording medium. FIG. 14 shows the structure of an exemplary recording medium described in patent document 1. The exemplary recording medium includes multiple recording layers 3 and one tracking layer (control layer) 5 for the multiple recording layers 3.
Patent document 2 discloses a recording medium made by stacking control layers used for tracking and layers made of a photosensitive material. Patent document 3 discloses an optical memory device in which a recording layer is provided next to a core layer made of resin or a clad layer made of resin and a barrier layer is provided between the recording layer and the core layer or the clad layer to prevent them from blending.
Also, patent document 4 discloses a multilayer optical recording medium made by stacking recording layers and non-recording layers alternately using adhesive sheets each made up of an optical recording layer containing a photosensitive material and an adhesive layer.
[Patent document 1] Japanese Patent No. 3110532
[Patent document 2] Japanese Patent Application Publication No. 2003-36537
[Patent document 3] Japanese Patent Application Publication No. 2003-141739
[Patent document 4] Japanese Patent Application Publication No. 2005-259192
However, in a multilayer optical information recording medium having a structure as described above, as the number of layers increases, fluctuation of reflectance caused by differences in the wavelength or incidence angle of light becomes greater and wavelength dependence becomes greater. In other words, the amount of reflected light in recording or reading a signal fluctuates depending on the wavelength or incidence angle of the light. This fluctuation causes an increase in noise and results in a decrease in the S/N ratio.
Also, in a multilayer optical recording medium with a control layer used for tracking as shown in FIG. 14, as the number of recording layers increases, the distance between each recording layer and the control layer increases. This makes it difficult to position a laser beam accurately.
Further, since the recording layers are formed just above the control layer, methods that can be used to process the control layer are limited and therefore flexibility in designing the control layer is reduced.
Meanwhile, in recent years, with the development of digital technologies and the improvement in data compression techniques, optical disks such as a digital versatile disk (DVD) have gotten a lot of attention as media for recording information such as music, movies, photographs, and computer programs (hereafter, may also be called “contents”). Also, as the prices of optical disks become lower, optical drives for recording and/or reproducing information on optical disks have become widespread.
As the data sizes of contents increase year by year, there is an increasing demand for an optical disk with higher storage capacity. One way to increase the storage capacity of an optical disk is to provide multiple recording layers. Currently, development of optical disks having multiple recording layers (hereafter, may also be called “multilayer disks” or “multilayer optical disks”) and optical drives for recording/reproducing information on such multilayer disks are very active.
However, if the number of recording layers in a conventional optical disk is increased, the amount of light reflected from a recording layer decreases as the distance between the recording layer and the incidence plane increases, because light is absorbed by other recording layers. As a result, the amount of light reflected from a distant recording layer decreases to such a level that it is difficult to detect a signal. Also, a conventional laser diode may not be powerful enough to record information on such a multilayer optical disk. These problems have been limiting the number of recording layers in an optical disk.
To solve the above problems and thereby to increase the number of recording layers, multilayer disks using two-photon absorption materials have been proposed (see, for example, patent documents 5 and 6). The refractive index of a two-photon absorption material changes when it absorbs two photons simultaneously. The proposed multilayer disks utilize this characteristic of two-photon absorption materials. On a proposed multilayer disk, information is recorded by changing the refractive index of target areas. These refractive index changed areas are called pits. More specifically, information is represented by the lengths and combination of refractive index changed areas and refractive index unchanged areas.
The probability of occurrence of two-photon absorption is proportional to the square of an applied optical-electric field (intensity of an incident light). Therefore, two-photon absorption occurs only in an area where the energy of an incident light is concentrated. When an incident light is focused by a lens, two-photon absorption occurs only around the focal point and does not occur in other areas where the incident light is not focused. In other words, the refractive indices of recording layers other than that on which incident light is focused do not change and those recording layers transmit the incident light without changing its intensity. Therefore, in this case, increasing the number of recording layers does not make it difficult to detect a signal or cause recording power shortage problems.
Thus, using two-photon absorption materials makes it possible to increase the number of recording layers and thereby to greatly increase the storage capacity of an optical disk. However, as in the case of conventional multilayer disks, forming guide tracks on each of the recording layers results in increased costs.
To obviate this problem, multilayer disks having guide tracks on a layer other than recording layers have been proposed (see, for example, patent documents 7 and 8).
Patent document 7 discloses a recording medium having a servo layer. With the disclosed recording medium, servo control is performed by detecting reflected light from the servo layer. However, if the recording medium is tilted in the radial direction in relation to the incidence angle of light, a tracking error may occur on a data layer that is distant from the servo layer. For example, on a data layer that is 1 mm distant from the servo layer, when the recording medium is tilted 1 degree in relation to the incidence angle of the light, the focal point of the light is shifted as much as 17.4 μm. On a Blu-ray disk with a track pitch of 0.32 μm, 17.4 μm is equivalent to about 50 tracks. For this reason, the recording medium disclosed in patent document 7 requires a tilt control that is different from that for a recording medium with a few recording layers. Also, although a small light spot can be formed on a data layer where a two-photon absorption material is used, a light spot becomes large on the servo layer where no two-photon absorption material is used. This problem makes it difficult to increase the track density of a recording medium and thereby makes it difficult to increase the storage capacity per data layer.
Patent document 8 discloses an optical information recording medium including a first layer having alternate convexities and concavities and a second layer having alternate convexities and concavities. In this case, however, it is very difficult to accurately align the convexities and concavities on the first and second layers.
[Patent document 5] Japanese Patent Application Publication No. 6-28672
[Patent document 6] Japanese Patent Application Publication No. 2004-79121
[Patent document 7] Japanese Patent Application Publication No. 2002-312958
[Patent document 8] Japanese Patent Application Publication No. 2005-18852